A key feature of several anti-HIV neutralizing antibodies is the unusually long length of their CDR H3s. Cow antibodies are unique in having ultralong CDR3 regions that can be over 60 amino acids in length and are cysteine-rich. Crystal structures of two different antibody Fab fragments reveal that these CDR3s form a very unusual architecture composed of a long ?-strand stalk which supports a disulfide rich knob that protrudes far from the immunoglobulin surface. Interestingly, different antibodies contain different patterns of disulfides, which result in different knob structures. Deep sequencing reveals extensive diversity in the ultralong CDR3s where a multitude of different disulfides could potentially form within the knob. Analysis of clonally derived sequences suggests that this diversity results from somatic hypermutation of an ultralong germline D region that has a severe codon bias towards mutation to cysteine. Thus, the bovine antibody system may produce an unprecedented repertoire of mega CDR3s that fold into an impressive diversity of minifolds containing combinations of somatically generated disulfides. We have immunized cows using the BG505 gp140 trimer and found that they are capable of making a robust and broadly neutralizing serum antibody response. In this exploratory proposal, we will take advantage of the unusual ultralong cow antibody repertoire to generate new monoclonal antibodies against HIV gp120 with the goal of identifying new neutralizing epitopes on HIV.